The present invention relates to a laser system for trimming or severing thin-film resistors fabricated on an undoped gallium arsenide substrate. The present invention also relates to a laser-trimmable resistor network suitable for use in bias circuits for power amplifiers.
Many integrated circuits, such as bias circuits for power amplifiers, include individual resistors that are required to have a specific value to achieve a desired circuit performance level. According to U.S. Pat. No. 4,782,320 issued to Shier, in applications of these types, the accuracies of individual resistors prior to trimming are typically on the order of only 15-20%, because there are wide manufacturing variations in the sheet resistance of the integrated circuit. These single resistors must then be trimmed using off-chip resistors to accurately reach a predetermined resistance value.
Other circuits do not require that an absolute value of a resistor be obtained, but rather require that two resistors be accurately matched in value, one relative to the other. Creating a closely-matched pair of resistors is easier than creating an individual resistor with a certain resistance value, because process variations in the former affect both matched resistors equally. Thus, the level of matching of integrated circuit resistors that is achievable by controlling parameters of the manufacturing process is approximately 0.1-0.3%, as described by Shier. But for some circuits, such as analog-to-digital converters, even this degree of precision is inadequate.
In order to achieve a higher level of precision than that achievable by fabrication processes, it is known in the art to use a laser to trim a thin-film resistor fabricated on a silicon substrate. The laser alters the shape of a resistor and thereby brings its resistance to a desired value. Alternatively, the resistor may be severed altogether, if used as part of a resistor trimming network.
A conventional laser-trimming system is shown in FIG. 1. An integrated circuit 10 includes a trimmable resistor 20 and other circuit elements (not shown) that are fabricated on a silicon substrate. Resistor 20 is typically made from a resistive thin-film material, such as nickel chromide, tantalum nitride, cesium silicide, disilicide, and polycide.
Integrated circuit 10 is coupled to an automatic test system 50 that measures the electrical properties of integrated circuit 10 in its untrimmed state. In response to the measured electrical properties, automatic test system 50 computes a desired trimming resistance. A predetermined trim pattern is obtained from memory 60 and provided to the laser drive and control 40. In response, laser drive and control 40 positions laser 30 at desired positions over integrated circuit 10 and actuates the laser to produce a radiation beam that is focused on a predetermined area of the trimmable resistor 20.
The wavelength of this radiation beam is typically selected based upon the light absorption characteristics of the silicon substrate and the resistive material. At certain wavelengths (for example, 1.32 xcexcm), the silicon substrate is almost transparent to the beam, while resistor 20 absorbs it. Thus, at these wavelengths, portions of resistor 20 may be vaporized without causing damage to the silicon substrate.
The laser-trimming technique described in the above paragraphs has been successfully employed to trim resistors fabricated on a silicon substrate. Clearly, it would be very desirable to employ the technique to trim resistors fabricated on a gallium arsenide substrate as well. But to the inventors"" knowledge, no one had successfully done so at the time of the present invention.
In fact, only one reference has been found that mentions laser trimming of resistors fabricated on gallium arsenide: U.S. Pat. No. 5,569,398 issued to Sun et al. This reference suggests that resistors fabricated on gallium arsenide may be trimmed using a laser with an output wavelength within the range from 1.0 to 3.0 xcexcm. Sun et al. derives this wavelength range (1.0 to 3.0 xcexcm) by: (1) identifying wavelengths at which gallium arsenide is believed not to absorb laser light (those wavelengths above 1.0 xcexcm); (2) identifying the wavelengths at which metallic thin-film resistive materials (such as platinum, nickel, tungsten, and aluminum) are known to absorb laser light (about 0.0 to 3.0 xcexcm); and (3) comparing the former and the latter wavelength ranges to obtain a range in which the gallium arsenide substrate does not absorb laser light, while the metallic resistive material does: 1.0 to 3.0 xcexcm.
Sun et al.""s suggestion that gallium arsenide does not absorb laser light having a wavelength from 1.0 xcexcm to 3.0 xcexcm is also supported by the experimental results of W. G. Spitzer and J. M. Whelan, as published in Physics Review, 114, 1 (1959) 59-63 and reproduced herein as FIG. 2. Spitzer and Whelan studied the relationship between the optical absorption characteristics of gallium arsenide and the doping level in gallium arsenide, for various wavelengths. Specifically, they showed that a strong correlation exists between the conduction electron concentrations induced by various levels of doping and the optical absorption coefficient. (The term xe2x80x9coptical absorption coefficientxe2x80x9d is defined as a unit of measure of the attenuation caused by the absorption of energy that results from its passage through a medium. Absorption coefficients are usually expressed in units of reciprocal distance. See Terms and Definitions, MIL-STD-2196 (SH), Glossary, Fiber Optics (1989).)
For every doping level tested by Spitzer and Whelan, the optical absorption coefficient was found to be at a local minimum over a wavelength range from about 1.0 to about 4.0 xcexcm. For example, the absorption coefficient for a conduction electron concentration of 4.9xc2x71017 cmxe2x88x923 is fairly constant at about 3.0 cmxe2x88x921 within the wavelength range from 1.0 to 4.0 um. Similarly, the absorption coefficient for a conduction electron concentration of 1.3xc2x71017 cmxe2x88x923 is fairly constant at about 0.5 cmxe2x88x921 within the wavelength range from 1.2 to 4.1 um. Thus, based on the results of Spitzer and Whelan, one would expect that as the doping level is decreased, the optical absorption coefficient correspondingly decreases, and the lowest optical absorption coefficient would be obtained when the gallium arsenide was completely undoped.
One would also expect from the results of Spitzer and Whelan that for undoped gallium arsenide, the absorption coefficient would remain at a local minimum throughout the range from about 1.0 to 4.0 xcexcm. One would expect, accordingly, that any wavelength within the range suggested by Sun et al. (1.0 to 3.0 xcexcm) would, in fact, be suitable for trimming a resistor fabricated on undoped gallium arsenide. But the inventors of the present invention have found that this is not the case. Through a series of experiments carried out under their direction, they have found, rather, that the critical range of laser wavelengths at which undoped gallium arsenide does not absorb laser light is much narrower: about 0.9 xcexcm to about 1.5 xcexcm. The inventors found that trimming with laser light having a wavelength shorter than 0.9 xcexcm or longer than 1.6 xcexcm caused damage to the gallium arsenide substrate. The inventors also found that trimming with laser light having a wavelength of 1.047 xcexcm produced the best results. These results were wholly unexpected in light of the teachings of Sun et al. and Spitzer and Whelan.
(The inventors note that their experiments were, in fact, carried out using gallium arsenide that was carbon-doped to a carbon concentration of between 1.0xc2x71015 cmxe2x88x923 and 5.0xc2x71015 cmxe2x88x923. Without this weak n-type doping, deep-level donors (impurities) in the gallium arsenide substrate would have caused the substrate to be slightly p-type. The substrate would thus have been slightly conductive, rather than semi-insulating. The effect of the carbon doping was to raise the conduction electron concentration of the gallium arsenide to about 1.0xc2x7107 cmxe2x88x923, which is about the same as pure, undoped gallium arsenide. The carbon doping is believed to have had no significant effect on the absorption characteristics of the gallium arsenide. Accordingly, the term xe2x80x9cundoped gallium arsenidexe2x80x9d is used herein to include all gallium arsenide substrates that are substantially semi-insulating and to exclude all substrates that are substantially conductive, such as those tested by Spitzer and Whelan.)
In addition to the problem described abovexe2x80x94finding a wavelength that is suitable to trim resistors fabricated on undoped gallium arsenidexe2x80x94the inventors had to overcome a second problem: the prior art does not disclose a laser output power that is suitable to trim a resistor fabricated on undoped gallium arsenide. A very low doping level allows the gallium arsenide to transmit laser light rather than absorb it, and thus the laser output power can be relatively high without damaging the gallium arsenide. But if the output power is too high, the gallium arsenide will be damaged by the laser light. Thus, the selection of an appropriate power level is not a trivial problem.
To summarize, although the desirability of laser trimming resistors fabricated on gallium arsenide is clear, no prior art reference known to the inventors teaches how to accomplish this feat. Specifically, there is a need to know: (1) the wavelength range at which laser trimming may be carried out; and (2) the laser power needed, based on the doping level in the gallium arsenide, to vaporize the resistor without damaging the gallium arsenide.
It is further known in the art to replace resistor 20 of FIG. 1 with a network of identical resistor links similar to those shown in FIG. 1 of Shier (reproduced herein as FIG. 3). With reference to FIG. 3, some of resistor links R1-R17 may be severed by laser in a predetermined pattern to produce a desired equivalent resistance of the network. The network described by Shier, for example, provides network resistances from 1.23 ohms to 9.0 ohms, albeit in two hundred irregular intervals that range from as small 0.0002 ohms to as large as 1.25 ohms.
But the network described by Shier is unsatisfactory for use in trimming bias circuits for power amplifiers for three reasons. First, the network of Shier requires seventeen resistors, each of which takes up space (die area) on integrated circuit 10. It would be highly desirable to reduce the number of resistors that are required in the network, and thereby to reduce the total area required on integrated circuit 10 by the resistor network.
Second, the range described by Shier (1.23 to 9.0 ohms) is small. A typical transistor amplifier requires a quiescent drain-source current of about 130 mA in order to operate with a desired linearity. In order to reliably establish this quiescent current, trimming network resistances must be provided that are adjustable in value from 50 to 1050 ohms.
Third, the resolution of the network of Shier varies greatly. For example, the smallest increase in resistance values in Shier is 0.0001 ohms (from 1.3013 to 1.3014 ohms), while the largest is 1.25 ohms (from 7.75 to 9.0 ohms). This resolution is both too high (for resistances from 1.23 to about 6.0 ohms) and too low (for resistances from 6.0 to 9.0 ohms). That is, the resolution achieved in the range of resistances from 1.23 to about 6.0 ohms is higher than is necessary to produce a quiescent current with an acceptable accuracy. This high resolution is gained at the cost of using many resistors, each of which requires die space on integrated circuit 10. On the other hand, the resolution achieved in the range of resistances from about 6.0 to 9.0 ohms is too low: the quiescent current in this case is insufficiently controlled, and it is possible for the quiescent current to vary outside design limits. Accordingly, a resistor network for trimming bias circuits for power amplifiers is needed that will provide a constant resolution, such as 50 ohms, over its entire resistance range from 50 ohms to 1050 ohms.
Accordingly, an object of the invention is to provide a laser system and method for cleanly processing a thin-film resistor fabricated on an undoped gallium arsenide substrate, without damaging or affecting adjacent circuit elements or the underlying or surrounding substrate. Another object of the invention is to provide a thin-film resistor that is fabricated on an undoped gallium arsenide substrate and that may be laser trimmed. A further object of the invention is to provide a laser-trimmable resistor network that may be used to trim a bias circuit for a power amplifier to obtain a desired quiescent current in the amplifier.
The present invention is directed to a laser system and method for cleanly trimming or severing resistors fabricated on an undoped gallium arsenide substrate without damaging or affecting adjacent circuit elements or the underlying or surrounding substrate. The system comprises the following elements: (1) a laser source adapted to generate an output at a wavelength within the range of 0.9 to 1.5 xcexcm; (2) a laser-trimmable resistor formed on a gallium arsenide substrate; and (3) a beam positioner and alignment system to align the laser source with the target structure. Preferably, the laser source produces an output within a wavelength range of 0.9 to 1.1 xcexcm, or still more preferably, of about 1.047 xcexcm.
The invention is also directed to a method for trimming a resistor fabricated on a gallium arsenide substrate. The method comprises: (1) generating a laser output at a wavelength in a range of about 0.9 to 1.5 xcexcm; and (2) directing the laser output to illuminate the target resistor. Preferably, the laser output power is within the range from 0.25 to 0.45 xcexcJ. More preferably, the laser output power is about 0.35 xcexcJ.
The invention is further directed to a thin-film resistor suitable for modification by laser, comprising: (1) a substrate of undoped gallium arsenide; (2) a first layer of protective dielectric; and (3) a layer of NiCr. Preferably, a second layer of protective dielectric lies upon the layer of NiCr. In an alternative embodiment, nickel or another thin-film resistive material may be substituted for NiCr. The thicknesses of these layers are preferably as follows: (a) substrate, about 500 xcexcm; (b) first dielectric, about 840 angstroms (xc3x85); (c) NiCr, about 430 xc3x85; and (d) second dielectric, about 940 xc3x85.
Electrical connections to a trimmable resistor having the above structure may be made via a metallization layer located at the ends of the resistor. The metallization layer may contact the NiCr from either side or from above.
The invention is further directed to a laser-trimmable resistive network suitable for trimming a bias circuit for a power amplifier. The resistor network comprises: (1) first and second resistive-film resistors in parallel; (2) a third resistive-film resistor joining one end of said first resistive-film resistor with the corresponding end of said second resistive-film resistor; (3) a fourth resistive-film resistors connected at one end to the node formed by said first resistive-film resistor and said third resistive-film resistor; and (4) a fifth resistive-film resistors connected at one end to the node formed by said second resistive-film resistor and said third resistive-film resistor. Preferably, the first resistive-film resistor has a resistance which is less than one of said fourth resistive-film resistor and said fifth resistive-film resistor. Still more preferably, the first resistive-film resistor has a resistance which is no more than half of the resistance of one of said fourth resistive-film resistor and said fifth resistive-film resistor.